Although various ideas and modes of apparatus have been proposed to prepare oil from waste plastics, it is a matter of fact that none of them have been operationally practicable.
By way of example, there is a technique of applying the mechanism of an extrusion molding machine, the technique comprising continuously conducting thermal decomposition of waste plastics into oil under heat while continuously carrying them by a continuous carrying means like a screw conveyer which Is characteristic of the extrusion molding machine. In applying the mechanism of the extrusion molding machine, the extrusion molding machine, being a highly established technology, has been highly expected that a resultant machine in a compact form will have a relatively large throughput, and will permit continuous automatic processing. Therefore, many researches and developments have been conducted all over the world, but no machine has been yet put into practice as mentioned before.
One of the major reasons why the conventional technology of preparing oil including the method of extrusion molding is the evolution of a large quantity of carbon in the decomposition process. Carbon generated in large quantity will attach to the inner wall of a decomposition reactor and thereby inhibits heat transfer and renders a stable control of the decomposition reaction difficult. As a result, recovered products having a desired composition cannot be efficiently obtained. Furthermore, since a great deal of efforts and time are required and the reaction processes become increasingly dangerous, many supervisory personnel are required. As a result, the machine will be economically poor and, therefore, cannot be operated as a practical machine.
In addition, insufficient controllability of the recovered products is also an obstacle to practical use. Although the recovered products preferably has a constituent composition suitable as a fuel oil for industrial burners etc., the recovered products by conventional techniques have a reduced quality due to contamination of carbon, or not suitable as a fuel oil due to contamination of too much gasoline derived from over-decomposition. It is, therefore, impossible to enhance the added value for practical operation of a system, and thus the recovered products cannot be applied for the operation of a practical system.
In order to prevent evolution of carbon and to control the composition of the recovered products which are required for realizing practical operation of preparing oil, exact knowledge on the mechanism of polymer decomposition and carbon evolution and proper countermeasures based on the information should be required. In this regard, however, the conventional technologies have been insufficient, and they can neither effectively prevent the evolution of carbon nor effectively control the composition of the recovered products.
From the above standpoint, the inventor conducted a thorough research and analysis on the mechanism of polymer decomposition and carbon evolution, and obtained the following findings. First, the decomposition of a polymer into oil was found to take place in the following processes: the solid material is melt and liquefied, which is then heated in the liquefied state to decompose the higher structure of the polymer to a lower structure, and then at this state decomposition starts for the first time, which causes the evolution of gasified components having different molecular weights depending on different decomposition temperatures, and the gasified components is cooled to yield a recovered product having a fixed composition. The composition of the recovered product is most significantly affected by the control of the heating temperature and the decomposition reaction in the liquid state. Therefore, the control of the heating temperature and the decomposition reaction for the liquid state Is most important for the resultant composition of the recovered product.
Carbon is mostly generated when the gasified components evolved in decomposition, especially those in which low molecular weight components are more predominant, are excessively heated. All the conventional processes are conducted under the conditions in which the gasified components cannot be free from the excessive heating because they are contained or trapped in the solid materials and the liquefied components, which is the biggest reason for the generation of a large quantity of carbon. It is therefore most important that the gasified components evolved from the liquefied components be immediately separated from the latter so that the former may not be exposed to excessive heating.
Especially, in the conventional techniques employing a mechanism of the extrusion molding machine, the liquid components after liquefaction are conveyed in the compressed highly densed form due to the extrapolation of the idea in the molding field that the compressing power and the shear stress derived from the strong conveying by the screw should also be used as a heat source. As a result, the liquid components tend to be contained and cause the generation of a large quantity of carbon. Furthermore, since a thick layer is formed by a polymer having low thermal conductivity, the effective control of the temperature of its center becomes difficult, thus disenabling the effective control of the composition of the recovered product.